Method and device for controlling a turning operation

ABSTRACT

A device increases the surface smoothness of a turned surface. The device comprising a control system with a control unit and an actuator connectible to the control unit and connectible with a tool holder. The actuator in adapted to impart a vibrating motion in the lateral direction to the tool holder. A method will also increase the surface smoothness of a turned surface, comprising the step of controlling the vibrations of the tool holder during turning. The method also comprises the step of imparting a vibrating motion in the lateral direction to the tool holder. Moreover, a turning lathe and a turning tool holder which like the device are designed to generate vibrating motion in the lateral direction.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/SE99/01884 which has an Internationalfiling date of Oct. 19, 1999, which designated the United States ofAmerica and was published in English.

FIELD OF THE INVENTION

The present invention relates to a method and a device for controlling aturning operation, more specifically a method, a device, a turning toolholder and a turning lathe for increasing the surface smoothness of aturned surface.

BACKGROUND ART

When a workpiece is worked by means of a lathe, a certain degree ofunevenness always arises in the turned surface. The unevenness can beresembled to stripes or threads and arises owing to the cutting edge ofthe working tool having a limited nose radius. The tools aremanufactured with a plurality of different standard radii. The radius ofthe cutting edge results, in combination with the feeding, in a surfacewhich is not quite smooth. A low feeding speed certainly gives asmoother surface but is irrational in industrial manufacture andtherefore does not solve the problem.

For reasons of rationality and expense, much would be gained if, inspite of a relatively high feeding speed, it would be possible to obtaina surface having such a high smoothness that the finishing which todayis often necessary can be eliminated or, in any case, be significantlyreduced.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a devicefor increasing the surface smoothness in turning.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further advantages thereof will now be described inmore detail by way of embodiments which reference to the accompanyingdrawings which are given by way of illustration only, and thus are notlimitative of the present invention, and in which:

FIG. 1 is a schematic perspective view of an embodiment of the inventivedevice;

FIG. 2 is a schematic view of an embodiment of a tool holder accordingto the invention; and

FIG. 3 is a schematic plan view of the device in FIG. 1.

DESCRIPTION OF AN EMBODIMENT

FIG. 1 illustrates essentially an embodiment of the device and also ofthe tool holder according to the invention. Reference numeral 1indicates a workpiece which is arranged in a lathe and rotates in thedirection indicated by arrow P1. The workpiece 1 is worked by means of atool 3, here referred to as insert, which is rigidly connected to a toolholder 5, here referred to as insert holder. The device comprises acontrol system with a control unit 7 and two actuators 9, 11, one ofwhich is indicated by dashed lines in FIG. 1 and both of which are shownin FIG. 2, which illustrates the actual tool holder 5 in a differentview.

Each actuator 9, 11 comprises an active element 9, 11, which here is apiezoceramic element. A piezoceramic element can in turn be designed asa unit or advantageously be made up as a so-called stack and/or ofseveral partial elements. Thus the element can be a solid body or aplurality of individual, but composed and interacting bodies. The activeelements 9, 11 are embedded in the body of the tool holder 5, which isalso referred to as shaft. More specifically, they are fixed by casting.The casting is carried out by forming for each active element 9, 11 arecess in the body of the tool holder, whereupon the active element 9,11 is placed therein and covered by casting. The active element 25, 27is glued preferably to the bottom surface of the recess. The activeelements 9, 11 are embedded fairly close to the surface of the toolholder 5, i.e. close to its lateral surfaces 5 d, 5 e. Moreover, theactive elements 9, 11 are plate-shaped and are oppositely arranged inparallel. The active elements 9, 11 are arranged on each side of thecentre axis of the tool holder 5, said centre axis being designated I—Iin FIG. 2. An active element 9, 11 is characterised in that it changesdimension when an electric voltage is applied across the same. Thedimensional change is related to the voltage. Moreover, the tool 3 ismounted on the upper side 5 c of the holder 5.

The control unit 7 is via a conduit 15 and a terminal 17 connected tothe tool holder 5. Inside, i.e. embedded in, the tool holder 5 extendto/from the terminal 17 conductors 30–33 of the active elements, or thepiezoceramic elements 9, 11, see FIG. 3. The piezoceramic elements 9, 11are elongate in the longitudinal direction of the tool holder 5, and theconductors 30–33, which are connected in pairs to a piezoceramic element9, 11 each, are connected to the front ends 11 a, 9 a and rear ends 11b, 9 b thereof.

The device operates as follows. The tool 3 and the tool holder 5 are fedin the direction of arrow P2 at a certain feeding speed M. The workpiecerotates in the direction of arrow P1 at a certain cutting speed. Thecombination of M>O, and the edge of the tool 3 having a radius causesremaining, helically extending ridges on the worked surface. More thananything, the ridges resemble stripes. The control unit 7 feeds controlvoltages to the actuators, more specifically to the piezoceramicelements 9, 11. When voltage is applied to the piezoceramic elements 9,11, they are thus extended to a greater or smaller degree depending onthe amplitudes of the voltages. In other words, each piezoceramicelement 9, 11 obtains a dimensional change in its longitudinaldirection, which also is the longitudinal direction of the tool holder5. The piezoceramic elements 9, 11 are preferably embedded in the toolholder 5 so that their boundary surfaces abut directly against thematerial of the body of the tool holder 5. The piezoceramic elements 9,11 have opposite power-transmitting surfaces in the form of their endfaces at the ends 9 a, 9 b, 11 a and 11 b. The end faces transfer thelongitudinal changes of the piezoceramic elements 9, 11 in the body ofthe tool holder 5. Since the piezoceramic elements 9, 11 are spaced fromthe centre axis I—I of the tool holder 5, the longitudinal changesgenerate turning moments which in the illustrated arrangement of thepiezoceramic elements 9, 11 show themselves as bending. By theexpression “spaced from the centre axis” is meant that the centre axesof the piezoceramic elements 9, 11 do not coincide with the centre axisof the tool holder 5. If the centre axes should coincide, no bendingmoment would be obtained, but merely a pure longitudinal change of thetool holder 5. The same would apply if the two piezoceramic elements 9,11 should be longitudinally changed concurrently and to the same extent.The forces induced by means of the piezoceramic elements 9, 11 bend thefront end 5 a of the tool holder 5 in the lateral direction, from sideto side, thanks to the control voltages to the respective piezoceramicelements 9, 11 being applied so that the piezoceramic elements 9, 11 arelongitudinally changed in opposition to each other. Thus the tool holder5 is made to move in a vibrating manner alternatingly in and against thedirection of feed.

The turning moments thus act about an axis which is perpendicular to thecentre axis I—I and produce a vibrating motion in the lateral direction,as indicated by arrow P3. By the lateral vibrations, the groove whichthe tool forms in the surface of the workpiece 1 is widened and thestripes are worked off. The appearance of the control voltages, however,is important to the result. In a preferred embodiment of the device, thecontrol unit 7 generates composite control voltages having a wide,noise-like frequency content. A factor in this context, however, is thefeeding speed M which may vary quite considerably between differentturning operations. The feeding speed is above all important to theamplitude of the control voltages. A preferred embodiment of theinventive device therefore comprises a control unit which is adjustablein respect of the amplitude of the control voltages. As a result,different amplitudes can be generated.

Alternative Embodiments

The above specification essentially constitutes a non-limiting exampleof how the device according to the invention can be designed. Manymodifications are possible within the scope of the invention as definedin the appended claims. Below follow some examples of suchmodifications.

In an alternative embodiment, the control unit also comprises a meansfor adjusting the frequency content of the control voltages.

In a further alternative embodiment, the control unit has preset valuesof frequency and amplitude of the control voltages.

In one more alternative embodiment of the inventive device, the controlunit 7 operates with fed-back control, which means that it strives toset the amplitude of the vibrations at a suitable level by means offeed-back from sensors. The control unit 7 can be selected among manydifferent types, such as analog fed-back control unit, conventional PIDregulator, adaptive regulator or some other suitable type of controlunit. To achieve said fed-back control, the sensors 13, 15 are arrangedin the tool holder 5 as illustrated in the Figures. The sensors 13, 15are arranged in front of the actuators 9, 11. By “in front of” is meantcloser to the end of the tool holder 5 where the tool 3 is mounted, saidend being naturally considered the front end 5 a of the tool holder 5.The opposite end 5 b thus is the rear end of the tool holder 5. Thesensors 13, 15 consist of piezoelectric crystals which generate anelectric voltage when subjected to forces. The sensors 13, 15 arepreferably, like the actuators 9, 11, embedded in the body of the toolholder 5 and are electrically connected with the control unit 7 viaconductors which are connected in the same way as the conductors 30–33of the actuators, but which for reasons of clarity are not shown.

The sensors 13, 15 are subjected to alternating pulling and pressingforces. Each sensor 13, 15 then generates a sensor voltage which variesconcurrently with the variations in force. The sensor voltages aredetected and analysed by the control unit 7, which controls theactuators 9, 11 in accordance with the desired amplitude of the sensorvoltages. The regulation which this involves is carried out by means ofa control algorithm. A large number of known control algorithms areavailable.

In one more alternative embodiment of the device according to theinvention, the control unit takes the present feeding speed intoconsideration, i.e. the control unit has a means for indicating whichfeeding speed is appropriate for the turning operation which is tobegin. In an NC-controlled lathe, the means can even automaticallycollect this information directly from the NC control system.

A further possible modification is to change the number of actuators. Inthe simplest case, one actuator is arranged in the tool holder. Toachieve a more symmetric application of forces on the tool holder, it ishowever advantageous to arrange at least the above-described pair ofactuators in the described opposite arrangement. There is nothing toprevent that a larger number of actuators are arranged which areoppositely arranged in pairs in the tool holder. For practical reasonsand in view of the production costs, it is however disadvantageous toembed a large number of actuators.

The method of mounting the active elements may be varied. In addition tothe above-mentioned way of mounting, they can be, for example,premounted in a mould in which the tool holder is cast. If they arefixed by casting later, as has been described above, they can either becovered with the same material as that of which the tool holder is madeor with some other convenient material. Moreover it is possible to usealternatives to the above-described, preferred mounting, where theelements are certainly glued to the base of the recess but two oppositepower-transmitting surfaces essentially generate the turning moments.Such an alternative means that the dimensional change is completelytransferred via the glue joint, which in principle is possible withtoday's strongest adhesives. In that case, the abutment of theabove-mentioned power-transmitting surfaces can be omitted, whichreduces the claims for adaptation. Also other variants are containedwithin the scope of the invention.

The active elements are in respect of form not bound to be rectangularlyparallelepipedal and plate-shaped as the shown elements, but the formmay vary. The plate shape, however, is advantageous since it contributesto minimising the volume of the element. Moreover, an elongate form isan excellent property which also contributes to imparting to the elementa small volume. It is preferred that the dimensional changes occur inthe longitudinal direction of the element.

Basically, other types of actuators and ways of mounting than thosedescribed above are contained within the scope of the invention.However, embedded, active elements have obvious advantages.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A device for increasing surface smoothness of a turned surface, the device comprising a control system comprising a control unit and an actuator connectible to the control unit and connectible with a tool holder, the actuator being adapted to impart a vibrating motion in a lateral direction to the tool holder wherein the tool holder moves in a vibrating manner alternatingly in and against a direction of feed when the device is mounted in a turning lathe, the actuator being spaced from a center, longitudinal axis of the tool holder, wherein the actuator is at least one piezoceramic element which is movable in a direction parallel to the center, longitudinal axis of the tool holder in order to create side-to-side vibration motion in the tool holder.
 2. The device as claimed in claim 1, wherein the actuator comprises an active element which is embeddable in a body of the tool holder.
 3. The device as claimed in claim 1 or 2, wherein the control system comprises a vibration sensor connectible to the control unit and connectible with the tool holder, the vibration sensor detecting vibrations of the tool holder in the lateral direction, and the control unit controlling vibrating motion by controlling the actuator according to sensor signals from the vibration sensor.
 4. A turning tool holder comprising an actuator for imparting a vibrating motion in a lateral direction to the turning tool holder, the turning tool holder being moved in a vibrating manner alternatingly in and against a direction of feed when the tool holder is mounted in a turning lathe, the actuator being spaced from the center longitudinal axis of the tool holder, wherein the actuator is at least one piezoceramic element which is movable in a direction parallel to the center, longitudinal axis of the tool holder in order to create side-to-side vibration motion in the tool holder.
 5. The turning tool holder as claimed in claim 4, wherein the actuator comprises an active element embedded in a body of the turning tool holder.
 6. The turning tool holder as claimed in claim claim 4 or 5, wherein the actuator comprises at least one pair of active elements, the active elements included in the pair being oppositely arranged on each side of the center, longitudinal axis of the tool holder.
 7. The turning tool holder as claimed in claim 4, further comprising a vibration sensor embedded in a body of the turning tool holder.
 8. A turning lathe comprising a tool holder and an actuator connected with the tool holder, the actuator imparting a vibrating motion in a lateral direction to the tool holder in order to make the tool holder move in a vibrating manner alternatingly in and against a direction of feed, the actuator being spaced from a center, longitudinal axis of the tool holder, wherein the active element is a piezoceramic element which is movable in a direction parallel to the center, longitudinal axis of the tool holder in order to create side-to-side vibration motion in the tool holder.
 9. The turning lathe as claimed in claim 8, further comprising a control system, the control system comprises a control unit and a vibration sensor connected to the control unit and connected with the tool holder, the actuator being connected to the control unit, the vibration sensor detects vibrations of the tool holder in the lateral direction, the control unit controlling vibrating motion by controlling the actuator according to sensor signals from the vibration sensor.
 10. The turning lathe as claimed in claim 8 or 9, wherein the actuator comprises an active element which is embedded in a body of the tool holder.
 11. A method for increasing surface smoothness of a turned surface, comprising the steps of: controlling vibrations of a tool holder during turning, imparting a vibrating motion in a lateral direction to the tool holder in order to make the tool holder move in a vibrating manner alternatingly in and against a direction of feed, and providing an actuator in the tool holder, the actuator being spaced from a center, longitudinal axis of the tool holder and causing the vibrating motion, further comprising the steps of using a piezoceramic element as the actuator and moving the actuator in a direction parallel to the center, longitudinal axis of the tool holder.
 12. The method as claimed in claim 11, further comprising the step of controlling in a feed-back manner the vibrating motion by detecting lateral vibration of the tool holder and controlling the actuator according to the lateral vibration.
 13. The method as claimed in claim 11, further comprising the step of adjusting the vibrating motion in response to feeding speed. 